1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to an adjustable magnetic trip device which includes a tension spring for increasing the force required to operate the trip bar thereby extending the tripping range of the device.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Patent Nos. 4,503,408 and 4,697,163, assigned to the same assignee as the present invention. Such circuit breakers are used to protect electrical equipment, such as motors, from damage due to an overcurrent condition.
Such molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the separable main contacts may be opened by either an operating mechanism or by magnetic repulsion forces generated by a reverse electrical current loop formed between the upper and lower contact arms.
The separable main contacts are mechanically coupled to an operating mechanism which includes a cradle member having a latching surface. A latch assembly includes pivotally mounted latch levers adapted to engage the latching surface on the cradle member to latch the operating mechanism to close the separable main contacts. When the latch lever is disengaged from the latching surface on the cradle member, the operating mechanism causes the separable main contacts to be opened. A pivotally mounted trip bar engages the latch lever. In a normal position, the trip bar allows the latch lever to latch the operating mechanism. However, when the trip bar is rotated, the latch lever becomes disengaged from the latching surface on the cradle member to allow the operating mechanism to trip or open the separable main contacts.
Various means have been used to actuate the trip bar to cause the separable main contacts to be tripped or opened. For example, bimetallic elements have been used. These bimetallic elements are comprised of strips of dissimilar metals which separate at predetermined levels of electrical current. Such bimetallic elements are serially connected between the load and line side terminals of the circuit breaker assembly such that all of the current that passes through the circuit breaker passes through the bimetallic element. Bimetallic elements are generally used to protect electrical circuitry or electrical loads from an overcurrent condition, generally about 200-300% of the nominal current rating of the circuit breaker.
Another known means of actuating a trip bar is an electronic trip unit. Such devices include internal current transformers electrically coupled to electronic circuitry. The current transforms only allow a portion of the current flowing through the circuit breaker to flow through the tripping device. Electronic trip units are adjustable and may provide overload protection as well as short circuit protection, generally 1000% or more of the nominal current rating of the circuit breaker.
In yet other circuit breakers, a magnetic tripping device is provided. This tripping device actuates the operating mechanism in response to relatively high overcurrent conditions. Such magnetic tripping devices are serially coupled between the line and load side terminals of the circuit breaker. The magnetic tripping device includes a coil and a reciprocally mounted plunger assembly. The plunger assembly includes a plunger carried by a carrier having a hammer portion which engages the trip bar when the plunger is attracted downwardly by the coil. The plunger is biased upwardly by an operating spring during normal current conditions defining a magnetic air gap between the plunger and the coil. When the electrical current flowing through the circuit breaker is sufficiently high, the magnetic attraction forces are generated between the plunger and the coil to overcome the upward spring force on the plunger. This causes the plunger to be attracted downwardly until the hammer portion of the carrier strikes the trip bar causing it to rotate to allow the operating mechanism to unlatch and trip or open the separable main contacts. The magnitude of the electrical current required to cause the magnetic tripping device to engage and rotate the trip bar (electrical tripping current) is dependent upon the spring force of the operating spring, the magnetic air gap and the force required to rotate the trip bar.
In order to provide an adjustable trip range for such magnetic trip devices either the magnetic air gap is varied or the spring force of the operating spring is varied, or a combination of the two. An example of such a magnetic tripping device is disclosed in U.S. Patent No. 4,697,163, assigned to the same assignee as the present invention. In this magnetic tripping device, means are disclosed for varying the spring force on the plunger as well as varying the magnetic air gap. By varying the spring on the plunger and the magnetic air gap, the the magnitude of the electrical tripping current at which the magnetic tripping device actuates the trip bar can be varied. However there are practical limits on the amount of adjustment that can be provided. In applications of a relatively large electrical tripping current an operating spring having relatively higher spring force is utilized. However, such an assembly cannot be adjusted low enough such that the magnetic trip device can also be used for applications requiring a relatively low electrical tripping current. Thus, separate magnetic tripping devices are required in such applications.
Molded case circuit breakers with such magnetic tripping devices are used in a wide variety of applications, such as for motor protection. In such applications, the motor size and corresponding inrush current values may vary substantially. Thus, it is necessary to provide as wide of an adjustment range on the magnetic trip d as possible. Known magnetic trip devices the electrical tripping current of which is varied by either changing the air gap, varying the spring force on the plunger, or a combination of the two, have provided an insufficient adjustment range.